This invention relates in general to an apparatus and method for the clarification of water where the removal of suspended particles is accomplished by a flocculating agent and flotation using a stream of rising, microscopic air bubbles. More specifically, it relates to an improved clarification apparatus and method of the general type described in U.S. Pat. Nos. 4,626,345 and 4,931,175 using flotation combined with filtration through a bed of sand or the like at the bottom of the flotation tank.
Water clarification, the removal of particulate contaminants suspended in water, is used to treat waste water from manufacturing processes, particularly in the paper and pulp industries, in the treatment of municipal water drinking supplies, and in sewage treatment. The water can be clarified by sedimentation or flotation of the particles. Known sedimentation techniques and apparatus are efficient, but are limited even at peak efficiency to a comparatively slow sedimentation rate, about 0.5 gallons per minute per square foot. To handle large volumes of raw input water, sedimentation facilities must therefore be large, with attendant cost and space utilization disadvantages.
Flotation techniques dissolve a few percent of air by volume in pressurized water and then release the air in the form of microscopic bubbles which attach to the particles and carry them upwardly to the surface where they form a floating sludge. The particles are usually coagulated and flocculated using conventional agents such as alum and polymers before the air bubbles are introduced. Flotation techniques are theoretically capable of achieving clarification rates of 7.5 gallons per minute per square foot of flotation area. Heretofore in practice the rates have been less than this theoretical value, but significantly better than for sedimentation techniques.
Applicant holds several U.S. patents for water clarification apparatus and processes, including U.S. Pat. Nos. 4,022,696; 4,377,485; 4,626,345; 4,184,967; and 4,931,175. In the '696 clarifier, sold under the trade designations "SPC" and "Supracell", the flotation occurs in a circular tank. The raw water is fed into the tank via a central pipe, a hydraulic joint, and an inlet pipe with multiple outlets immersed in the tank which rotates about the tank. The inlet flow is oriented opposite to the direction of rotation of the inlet pipe and is at a velocity with respect to the rotation rate such that the raw water has a net zero velocity as it enters the tank. The raw water inlet flow assembly and a scoop for removing the floated sludge are mounted on a carriage that rotates about the tank. The scoop is preferably of the type described in applicant's U.S. Pat. No. 4,184,967. The rate of rotation is set so that the floated particles will reach the surface of the water held in the tank in the time of one rotation. A good degree of clarification can be achieved with each rotation of the carriage using a comparatively shallow tank, e.g. 16-18 inches.
U.S. Pat. No. 4,377,485 discloses a later clarifier of the present application sold under the trade designations "SAF" and "Sandfloat", which utilizes the basic approach of the '696 invention, also operating with the net zero velocity principle, but also adding a second filtration stage with a set of wedge-shaped sand beds covering the bottom of the flotation tank. The water clarified by the flotation process flows through the underlying sand beds into a clarified water collecting chamber. A suction apparatus mounted on the carriage periodically backwashes each section of the filter in combination with a flow of backwash water introduced through the collection chamber. During clarification, when the flocs are formed, pressurized, aerated water is introduced to the open, lower end of the flocculation chamber via a manifold with multiple outlets. The bubbles carry the flocced particulates upwardly to form a floated sludge layer. The clarification rate is comparable to that of the '696 "Supracell" clarifier.
U.S. Pat. No. 4,626,345 discloses a clarifier, sold under the trade designations "SASF" and "Sandfloat-Sedifloat". It does not utilize the net zero velocity principle, but does employ a sand bed for two stage clarification. In this apparatus, the raw water enters a central cylindrical compartment, which acts as a hydraulic flocculator, and then flows over a dividing wall into a surrounding circular flotation tank. Aerated water is added via a fixed manifold. Rotating elements include a scoop remover for the floated sludge and a suction hopper that cleans the filter beds. The scoop is the same general type as used with the Supracell and Sandfloat units described above. This '345 SASF apparatus offers a two stage clarification with a more compact unit and at a lower cost than the SAF clarifier.
While the SASF clarifier has proven to be effective, compact, and comparatively low cost, it does not meet certain regulatory requirements that first filtrate water--the clarified water that is produced after the sand bed, or a portion of the sand bed, is backwashed--be separated from the clarified water produced without the turbidity of a recent backwashing. Another problem is that the SAF and SASF clarifiers use a suction hood that covers segments of the sand bed to draw off accumulated contaminants and sludge. The SAF design uses a backwash water directed through clarified water outlets; the SASF relies solely on the action of a suction pump applied to the sand bed by the hood. As a result the SASF design does not induce a strong agitation of the sand bed which is conducive to a thorough cleaning. Also, both the SAF and SASF arrangements for backwashing remove some of filter medium with the contaminants and sludge. In the SAF unit a cyclone separator recovers the sand. Media loss considerations, in part, limit the thoroughness of the cleaning and the overall efficiency of at least the SASF clarifier.
In the SASF clarifier the sand bed portions all feed clarified water to an underlying collection well. This well then drains to a clarified water outlet. The presence of this collection well means that the first filtrate water is mixed with clarified water and that the available space for the control hydraulic flocculator is limited (530 liters, or 140 gallons, being a typical capacity for a flocculating tank of a five foot diameter SASF clarifier). This volume results in comparatively short retention times (3.8 minutes at a 140 l/min flow rate) which limit the thoroughness of the flocculation, and hence the efficiency of the clarification. In the SAF unit, a slotted pipe is used to collect backwashed sludge, but it is located in the water, under a hood, and it is connected to a suction pump. Backwash water is supplied through chambers under the sand bed sections. Radial slotted pipes are also used in the SPC clarifier, but they are not used in any backwashing. They rotate in the flotation tank to collect clarified water; the SPC has no second stage sand filter and there is no backwashing.
The efficiency of all of these known clarifiers i also limited by the efficiency of producing microscopic bubbles of optimal size for flocculation. Ideally, the bubbles should have a diameter of 40 to 80 microns to achieve a rising velocity of 30 cm/min. Also, they should be as numerous as possible, and should exhibit as little coalescing as possible once they are formed.
A straightforward expedient to produce more bubbles is to dissolve more air into the water. However, too great a saturation of dissolved air in the water to be decompressed produces bubbles that are too large and that rise with a velocity that disturbs the optimal flotation process.
It is also known that dissolved air is not released efficiently simply by decompression of the water. Some air will remain dissolved. To release all or most of the dissolved air, there must be a strong shearing action on the pressurized water. In applicant's U.S. Pat. No. 4,931,175 pressurized water with dissolved air enters a flotation tank through a vertical tube with a central rod terminated in a round plate that is spaced from the end of the tube to provide an annular outlet. The shearing action on the pressurized water as it strikes the plate and is diverted sideways releases the bubbles more efficiently. However, the annular gap of this outlet is comparatively large, (a typical value is 3-5 mm). Also, it is varied to equalize flows to a number of outlets, not to optimize the size of the bubbles being produced. Further, the outlet opening is large enough that it will not become clogged with the particulate contaminants carried in the water. At least one unit using small discs and very small holes has been described, but the holes readily plug if the flow rate is large enough for a practical clarifier. To date no known decompression valve for pressurized water can efficiently release most of the dissolved air in bubbles of optimal size for the flotation process without serious plugging.
It is therefore a principal object of this invention to provide a two stage clarifier and method of operation where the first filtrate water is isolated from other clarified water collected from the clarifier.
Another principal object is to provide a pressurized water decompression valve and method of discharging the pressurized water with dissolved air that reliably produces microscopic air bubbles of an optimal size for flotation and with a high degree of efficiency.
Still another principal object of the invention is to provide a decompression valve that substantially reduces the amount of aerated pressurized water used and therefore the power consumed in aerating the water.
A further principal object of the invention is to prove a compact two stage clarifier and method of operation that has a higher capacity, can be operated with a higher head loss, and is more efficient than comparable known clarifiers.
Yet another advantage is to provide a two stage clarification apparatus and method with improved flotation rate and improved backwashing and media retention.
A further object of is to provide all of the foregoing advantages as well as an increased capacity hydraulic flocculator and an associated longer retention time.
Another object is to provide all of these advantages at a favorable cost of manufacture.